Ground-state dipole moments - calculate

The Wd estimated from the RASCI is compared with other theoretical calculations [46, 54] in Table V. The present DF estimate of Wd deviates by 21%(29%) from the SCF (RASSCF) estimate of Kozlov et al. [46] and by 17% from the semiempirical result of Kozlov and Labzovsky [54], while our RASCI result departs by 6% (3%) from the SCF-EO (RASSCF-EO) treatment of Kozlov et al. [46] and is in good agreement with the semi-empirical result of Kozlov and Labzovsky. At this juncture, we emphasize that our computed ground state dipole moment of BaF (p(. = 3.203 debyes) is also reasonably close to experiment p(. = 3.2 debyes (see Table 5 of Ref. 38). 

There have been no previous direct non-BO studies of the response of H2 and its isotopomers to electric fields. The ground-state dipole moment of HD has been determined experimentally by Nelson and Tabisz [81] to be 0.000345 a.u. There have been several theoretical studies of the dipole moment of HD, all within the BO approximation but including adiabatic corrections. The calculated values by Wolniewicz, 0.000329 [83], Ford and Browne, 0.000326 [82], and Thorson et al., 0.000334 [84], aU agree well with the experimental value, although they are all about 5% too small. This is an extremely difficult experiment to carry out, and because all theoretical studies agree on the value, it... 

We have performed a series of semiempirical quantum-mechanical calculations of the molecular hyperpolarzabilities using two different schemes the finite-field (FF), and the sum-over-state (SOS) methods. Under the FF method, the molecular ground state dipole moment fJ.g is calculated in the presence of a static electric field E. The tensor components of the molecular polarizability a and hyperpolarizability / are subsequently calculated by taking the appropriate first and second (finite-difference) derivatives of the ground state dipole moment with respect to the static field and using... 

We started the analysis of the sulfonyl group as an acceptor with the calculation of 4-methylsulfonylaniline (I), which is an analogue of p-nitroaniline (II), and their methylated derivatives (III, and IV, respectively). We have found that while the ground state dipole moments are comparable for the nitro and sulfone derivatives (Table I), the p coefficients are different in magnitude, and depend on the method of calculation (Table I). 

Since the hyperpolarizability of a given molecule is a function of the donor and the acceptor properties, and nature of the conjugation path between them, we turned to the biphenyl system and analyzed the 4-amino-4 -methylsulfonylbiphenyl (V). The calculated ground state dipole moment of this molecule is smaller than expected for such an increase in the distance between the donor and the acceptor. 

Our calculations predict only minor differences between the ground state dipole moments for molecules containing nitro electron acceptors versus those possessing methylsulfonyl. In contrast, the hyperpolarizabilities behave much differently, in that calculated J3Z for the aminonitrostilbenes is about twice that of the aminosulfonylstilbenes and the nitroanilines are more than 5 times more nonlinear than the sulfonylanilines. The hyperpolarizabilities appear to be very sensitive to the details of the electron donors-acceptor interaction and hence accentuate the differences in the a values for nitro and methylsulfonyl. 

The measurement of ground-state dipole moments may help to establish the validity of the theoretical calculations. We measured two representative compounds, X, and XI (see below), where diallyl derivatives were used to increase solubility in nonpolar solvents (e.g., CC14). 

In this study we have described theoretical calculations, syntheses, optical spectra, ground-state dipole moment measurements, and measurements of molecular second-order hyperpolarizability coefficients (/J) for new stilbene and azobenzene derivatives containing a methylsulfonyl group as the electron acceptor. We have shown that theoretical calculations can be used to predict the ratio of molecular hyperpolarizabilities between similar compounds, and that these gas phase calculations underestimate /J, probably as a result of the valence basis set used in the calculations. 

The ground-state dipole moments of BPHTs were calculated by evaluating the vector sum of the it moment (computed by the Pariser-Parr-Pople (PPP) method) and the a moment (from cr-bond moments) [17], a separate set of dipole moment values being obtained by the parametric method 3 (PM3) ([25] and references therein) (Table 5). By comparison, the calculated ground-state dipole moment values were considerably lower than the experimental ones. In some cases, the agreement between the experimental and calculated ground-state dipole moments was rather poor [17]. 

Figure 2. The peptide moiety showing charge distributions on each of four atoms. The result is a permanent (ground state) dipole moment indicated by the arrow with the head of the arrow pointing in the positive direction. On absorption of a photon, the electron distribution changes. This results in the atoms having a new distribution of charge and the excited state will have a different dipole moment. The difference dipole moment (between the ground and excited states) is called the electric transition dipole moment, /i,., and its magnitude can be calculated from the area of the absorption curve as shown in Figure 3.

Experimental and calculated ground-state dipole moments of selected pyrimidines and purines... 

In our PPP + o-moment calculations, we assume that the excited-state it-component (as obtained by the PPP calculation) is different from the ground-state dipole moment, whereas the o-moment remains unchanged (i.e., the same as in the ground state). This approach has worked very well for a number of different series of organic heterocycles. 

The dipole moment of 5,7-diazaazulene has been calculated by CNDOVSB, STO-3G and 4-3IG ab initio methods <89JCS(P2)103>. As would be expected, the presence of the electron-attracting nitrogen atoms means that it has a lower ground state dipole moment than either azulene or 1- and 2-azaanulenes. 

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